1. Field of the Invention
The present invention relates to a seat reclining apparatus which is installed in a seatback-angle adjustable reclining seat.
2. Description of the Related Art
A type of reclining apparatus including: a base plate (lower arm) which is fixed to a seat cushion frame; a ratchet plate (upper arm) which is fixed to a seatback frame and provided on the inner periphery thereof with an annular internal gear (toothed portion); and pawls (a plurality of pawls are often provided at different positions in the circumferential direction) which are supported to be movable relative to the base plate in radial directions of the base plate, wherein the reclining seat is brought into a locked state, in which the ratchet plate and the base plate are prevented from rotating relative to each other, by bringing the outer toothed portion which is formed on each pawl into mesh with the annular internal gear of the ratchet plate, is widely known in the art as a seat reclining apparatus provided between a seat cushion frame and a seatback frame of a reclining seat. Each pawl is biased in a direction (locking direction) to bring the outer toothed portion thereof into mesh with the internal gear of the ratchet plate by a biaser such as a spring, and the reclining seat is brought into an unlocked state by moving each pawl in a direction to disengage the pawl from the internal gear of the ratchet using a cam member or the like against the biasing force of the biaser.
In this type of seat reclining apparatus, guide surfaces which slidably support and guide the pawls are formed on the base plate, and the pawls move in radial direction of the base plate along the guide surfaces. Between each pawl and the associated guide surfaces is secured a minimum clearance allowing the pawl to slide on the guide surfaces, so that each pawl is prevented from tilting beyond this clearance by the associated guide surfaces. However, in the case where an excessive load in a rotational direction which exceeds a normally expected range is exerted on the pawls, a tilt or deflection occurs in the pawls and the cam member that supports the pawls, which causes a reduction in the degree of engagement between the outer toothed portion of the pawls and the annular internal gear of the ratchet plate. Accordingly, there is a possibility that a required locking strength will not be able to be maintained.
The seat reclining apparatus disclosed in Japanese Unexamined Patent Publication No. 2003-70583 has a reinforcing structure which restricts the tilting amount of each pawl. This reinforcing structure consists of grooves which are formed on the base plate in addition to the guide surfaces, and protrusions which are formed on the pawls to be inserted into the grooves of the base plate, and the tilting amount of each pawl is restricted by engagement of the protrusion of this pawl with the inner surface of the associated groove.